Abstract We have designed a Program project ? Sedentary Time and Aging Research (STAR) ? to provide more rigorous and comprehensive evidence on how best to interrupt sitting time to improve prospects for healthy aging. In particular, we are proposing a paradigm shift away from energy expenditure as the only mechanism for improving health outcomes to investigate behaviors such as brief sit-to-stand transitions that expend little energy but engage muscles, improve postural blood flow, and may impact physical functioning in older adults. Further, we will study multiple novel targeted outcomes, including outcomes intended to reflect a range of alternative mechanisms that are important for healthy aging. The role of the Administrative Core is to ensure the quality of the science and day-to-day operations across all the projects and cores. In particular, it will provide scientific leadership to support and facilitate the integration of the research Projects and Core, provide administrative leadership to support logistic and financial requirements, support development of the next generation of sedentary behavior researchers and coordinate the dissemination of findings to university, scientific, and lay communities. The STAR program will include 3 Projects and 2 Cores and study postmenopausal women at risk for chronic disease. STAR includes 2 randomized trials, in the laboratory (N=86) and in the real world (N=592), and new computational techniques applied to existing accelerometer data in a subsample (n>6000) from the Women's Health Initiative cohort. Projects will investigate the consequences of extended sitting, standing, brief sit-to-stand transitions and physical activity breaks on biomarkers of healthy aging including glucose regulation & blood pressure (all projects), endothelial functioning including femoral flow mediated dilation (Project 1), and changes in skeletal muscle mitochondrial respiratory capacity (Project 2). We will also investigate physical functioning and in Project 3, 5-7 year mortality risk. In addition, a Biostatistics Core will draw together the projects by investigating patterns across the different time points (acute, intermediary and long term) in the different projects. The Administrative Core will be co-led by 2 highly-experienced scientists with international reputations and demonstrated effectiveness in managing complex scientific enterprises, similar to the scale of the proposed Program Project. The co-leaders will be supported by an Executive Committee, distinguished Advisors, a DSMB, and an experienced Program Manager. The Administrative Core functions are designed to ensure the success of the financial, administrative, regulatory, scientific, training, and dissemination goals of the Program. Extensive but realistic meeting schedules are proposed to ensure effective interaction and strong coordination. The training activities are based on methods we have found to be successful previously, with some innovations and coordination with a new center on sitting in Latinas. The STAR program will provide a comprehensive, rigorous evidence base that will inform public health guidelines on interrupting sitting and healthy aging.